RFID tags

ABSTRACT

A system and method for the use of ultra high frequency RFID tags in conjunction with metal substrates, as well as substrates used to contain liquid. Briefly, an RFID isolator comprised of a material with complex magnetic permeability, used either by itself, or in combination with dielectric isolator material, is interposed between the RFID tag and the substrate. Alternatively, a material possessing at least two distinct dielectric constants is interposed between the RFID tag and the substrate, such that there is a high dielectric constant at the interface with the substrate, and a low dielectric constant at the interface with the RFID tag. This material can be a single material having a dielectric constant gradient, or alternatively, two or more separate layers, each with a uniform but different dielectric constant, sandwiched together.

This application claims priority of U.S. Provisional Application Ser.No. 60/615,826, filed Oct. 4, 2004, and U.S. Provisional ApplicationSer. No. 60/713,861, filed Sep. 2, 2005, the disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Radio Frequency Identifier (RFID) tags are used in a variety ofapplications, such as inventory control and security. Unlike barcodetracking systems, the advantage of these more intelligent RFID systemsis that an RFID system can store specific information about an articleand can read that information on a tag without requiring line of sightor a particular orientation. This means that RFID systems can be largelyautomated, reducing the need for manual scanning.

These RFID tags are typically placed on or in articles, or containerssuch as cardboard boxes. The RFID tags work in conjunction with an RFIDbase station. The base station supplies an electromagnetic wave output,which acts as the carrier frequency. Data are then used to modulate thecarrier frequency to transmit specific information. RFID systemstypically operate at either a low frequency range (generally less than100 MHz), or a higher frequency range (greater than 100 MHz). In manyapplications, one such higher frequency range is between 800 and 1000MHz (defined as the UHF Band), with 915 MHz being the most common highfrequency currently utilized in the United States. Most RFID systemsutilize frequency hopping centered around this frequency, such that theoverall frequency range is approximately 902 to 928 MHz. A second highfrequency used by RFID tags in the United States is 2450 MHz. Currently,European standards utilize 869 MHz and the Japanese standard is 953 MHz.

Many RFID tags contain integrated circuits, which are capable of storinginformation. Depending on the specific implementation of the RFID tag,the integrated circuit may be capable of replacing stored informationwith new information at a later time. When the base station requestsdata, the integrated circuit supplies the information that it has storedin response to that request. In those RFID tags that permit informationto be rewritten, the integrated circuit overwrites its existinginformation when new data are received from the base station.

In addition to the integrated circuit, the RFID tags contain an antenna.The antenna is needed to receive the electromagnetic waves generated bythe base station, and to transmit data via the same frequency. Theconfiguration of the antenna can vary, and includes flat coils, patches,microstrip antennas, stripline antennas and dipoles.

Some of these RFID tags are self-powered, that is, they contain aninternal power supply such as a battery. Other RFID tags arefield-powered. These latter tags use incident RF energy transmitted bythe base station to supply their required voltage. The RF energy isreceived by the tag antenna as an AC signal, which is then rectified toform a DC voltage, which is used to power the integrated circuit.

These integrated circuits have a minimum voltage requirement below whichthey cannot function and the tag cannot be read. The rectified DCvoltage is a function of the signal strength of the receivedelectromagnetic wave. For example, a RFID tag that is proximate to thebase station will receive more energy and therefore be able to supplysufficient voltage to its integrated circuit, as contrasted to a RFIDtag which is physically farther away from the base station. The maximumdistance between the base station and the RFID tag at which the RFID tagcan still be read is known as the read distance. Obviously, greater readdistances are beneficial to nearly all RFID applications.

One benefit of RFID tags that operate in the high frequency range is thepotential to have much greater read distances than tags operating at lowfrequency. RFID tags utilizing the 915 MHz frequency range typicallypossess a read distance in excess of 10 feet in free air. In contrast,lower frequency (such as 13.56 MHz, which is part of the HF Band) tagsrarely achieve read distances greater than 2 feet.

One reason for this difference is due to the difference in the energytransfer mechanisms at the HF and UHF frequencies. As described above,it is the electric field of the propagating signal that gives rise to apotential difference across the antenna at UHF frequencies. In contrast,passive RFID tag systems operating in the HF frequency band at 13.56 MHzemploy magnetic induction to couple the transponder tag and the reader.The power required to energize and activate the HF tag microchip isdrawn from the oscillatory magnetic field created by the reader.

Unfortunately, high frequency RFID tags cannot be read when the tag isin close proximity to a metal substrate or a substrate with high watercontent. Thus, an RFID tag attached to a metal container or to a bottlecontaining a soft drink cannot be read, from any distance.

Experimentation in the industry has shown that such RFID tags are onceagain readable if there is a substantial air gap interposed between thetag and the article substrate. This required air gap is typically atleast one quarter of an inch or greater. Various designs have beendeveloped to allow tags to “stand off” from the article substrate inorder to create this gap. However, standoff tags are impractical in themajority of commercial applications. The distance between the tag andthe article increases the likelihood of the tag being dislodged ordamaged in normal use.

Recognizing that an air gap acts as a dielectric insulator, tagmanufacturers have attempted to solve the stand off problem byinterposing a thin layer of a dielectric insulating material ofdielectric constant, k, between the tag and the article substrate. U.S.Pat. No. 6,329,915 discloses the use of a homogeneous material of highdielectric constant to address this issue. However, homogeneousmaterials with various k values have been tried with little or nosuccess.

Therefore, a system and method for allowing the use of RFID tags onthese substrates would represent a significant advance for the use ofhigh frequency RFID tags.

SUMMARY OF THE INVENTION

The present invention overcomes the shortcomings of the prior art.Specifically, the present invention provides a system and method for theuse of high frequency RFID tags in conjunction with substrates,particularly metal substrates, as well as substrates used to containliquid. Briefly, an RFID isolator comprised of a material with complexmagnetic permeability, used either by itself, or in combination withdielectric isolator material, is interposed between the RFID tag and thesubstrate. Alternatively, a material possessing at least two distinctdielectric constants is interposed between the RFID tag and thesubstrate, such that there is a high dielectric constant at theinterface with the substrate, and a low dielectric constant at theinterface with the RFID tag. This material can be a single materialhaving a dielectric constant gradient, or alternatively, two or moreseparate layers, each with a uniform but different dielectric constant,sandwiched together, This material overcomes the inability of many tagsto be read on metal substrates using prior art dielectric isolators. Inother cases, this material improves the read distance of tags that havelimited read distances with prior art dielectric isolators.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a view of a first antenna, tested in conjunction with thepresent invention;

FIG. 1 b is a graph depicting the read range characteristics of thefirst antenna;

FIG. 2 a is a view of a second antenna, tested in conjunction with thepresent invention;

FIG. 2 b is a graph depicting the read range characteristics of thesecond antenna;

FIG. 3 a is a view of a third antenna, tested in conjunction with thepresent invention;

FIG. 3 b is a graph depicting the read range characteristics of thethird antenna;

FIG. 4 a is a view of a fourth antenna, tested in conjunction with thepresent invention;

FIG. 4 b is a graph depicting the read range characteristics of thefourth antenna;

FIG. 5 a is a view of a fifth antenna, tested in conjunction with thepresent invention;

FIG. 6 shows a first embodiment of the present invention;

FIG. 7 shows a second embodiment of the present invention; and

FIG. 8 shows a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Passive UHF RFID tag antennas are optimized for use in free space or onlow dielectric materials, such as corrugated cardboard, pallet wood,etc. When a UHF RFID tag is in proximity to a metallic substrate, theimpedance of the tag antenna changes. For efficient power transmissionof the wave transmitted by the RFID reader to the IC circuitry of thetag, the antenna must provide a smooth impedance transformation fromfree space to the impedance of the IC circuitry. Antenna design usuallyassumes that the substrate with which the antenna will be in closeproximity has a magnetic permeability equal to unity. In substrates witha magnetic permeability not equal to unity, a parasitic inductance in anisolator material can be used to offset the parasitic capacitance of themetal substrate, thereby benefiting tag isolation. Alternatively,material of at least two distinct dielectric constants can be used tobetter isolate the tag from the metal substrate.

FIG. 6 shows a first embodiment of the present invention. RFID tag 100can be specially designed, or purchased from any of a number ofcompanies, such as Intermec Technologies Corporation, SymbolTechnologies (formerly Matrics Inc.), Alien Technology, PhilipsSemiconductor, and Texas Instruments. In the preferred embodiment, theRFID tag operates in the frequency range between 800 and 1000 MHz, withthe most preferably center frequencies being 869 MHz, 915 MHz and 953MHz. This RFID tag can be self-powered by inclusion of a power source,such as a battery. Alternatively, it can be field-powered, such that itgenerates its internal power by capturing the energy of theelectromagnetic waves being transmitted by the base station andconverting that energy into a DC voltage.

Article 110 is the object to be tagged. As described above, articlescomprising a metal substrate, or configured to contain liquid areproblematic, with respect to read distances. In various testing, a tagcould not be read when attached to a metal substrate. Recognizing thatthere is an interaction between the RFID tag 100 and the metalsubstrate, several designs have incorporated standoffs so as tointroduce a layer of air between the two components. While this improvesthe read distance of the RFID tag, it is commercially impractical due tothe likelihood of the RFID tag being dislodged or damaged. To simulatethe effect of an air gap, several manufactures have interposed a thinlayer of material 120 with a high dielectric constant. Unfortunately,the inclusion of a material with a high dielectric constant, so as toinsulate the tag from the metal, has shown little or no success.

Unexpectedly, favorable results were achieved by the present inventorswhen the article 110 and the RFID tag 100 were separated by a material120 possessing a dielectric constant gradient, such that the dielectricconstant at the interface between the material and the article 110 washigher than the dielectric constant at the interface between thematerial and the RFID tag 100. This gradient was ineffective, however,when the high dielectric constant was facing the RFID tag 100. Thematerial 120 may have one or both surfaces in contact with an adhesive,such as 3M Company silicone/acrylic double coated film tale 9731, tofacilitate adhering the material 120 to the RFID tag 100 and the article110.

Generally, the material 120 used can be an elastomer, a plastic or aceramic. The material includes a low loss dielectric filler, such astitanium dioxide, boron nitride, silicon dioxide, aluminosilicates,magnesium oxide, or aluminum oxides, to achieve the desired dielectricconstant. In the preferred embodiment, the material 120 is a siliconeelastomer polymer. Titanium dioxide is used to modify the dielectricconstant of the polymer. By mixing in titanium dioxide in a non-uniformmanner, it is possible to create a material having a high dielectricconstant on one surface and a lower dielectric constant on the oppositesurface. Another method that can be used to generate the dielectricgradient is to vibrate the mixture. Typically, the titanium dioxide hasa greater density than the base material. Therefore, by vibrating themixture, the titanium dioxide will tend to settle toward the bottom ofthe mixture, thereby creating a non-uniform distribution of thedielectric filler. In this example, the dielectric constant near thebottom will be higher than that at the top. Gradients produced within asingle layer are not limited to this embodiment. Gradients may be alinear, logarithmic, exponential or other non-linear function.

FIG. 7 shows an alternative embodiment of the present invention. Thedielectric gradient material was fabricated by sandwiching two layers ofvarying thickness, one of low dielectric constant and one of highdielectric constant. Top layer 220 and bottom layer 230 are placed incontact with each other, and are interposed between RFID tag 100 andarticle 110, such that the top surface of top layer 220 is in contactwith RFID tag 100 and the opposite surface of top layer 220 is incontact with bottom layer 230. Similarly, the top surface of bottomlayer 230 is in contact with layer 220 and the opposite layer of bottomlayer 230 is in contact with article 110. The dielectric constant of toplayer 220 is relatively low, preferably less than or equal to 4.0, whilethe dielectric constant of bottom layer 230 is relatively higher,preferably in the range of 8 to 35. In the preferred embodiment, the twolayers of material are pressed together. In an alternate embodiment, anadhesive, such as 3M Company silicone/acrylic double coated film tape9731, may be used to affix the two layers together. The presentinvention is not limited to the use of only two layers of material. Morethan two layers may be used to create the required dielectric constantgradient as shown below in Table 17. In this case, the gradient would bein discrete steps.

Gradients of both types, continuous (as shown in FIG. 6) or step-wise(as shown in FIG. 7), are both within the scope of the invention.

Also of interest in determining the proper configuration for top layer220 and bottom layer 230 is the thickness of each. The following tablesrepresent empirical data obtained using a Model X1020 tag manufacturedby Matrics, Inc. in conjunction with a metal substrate. The tag/metalsubstrate, with or without an isolator, was suspended inline with aMatrics RDR-001 reader and the combination was moved away from thereader until the tag could no longer be read. This distance was recordedas the read distance. The columns represent the thickness of bottomlayer 230 in inches, while the rows represent the thickness of top layer220 in inches. The values included in Table 1 are the recorded readdistances (in inches) at that particular configuration, where thedielectric constant of top layer 220 (k1) is 1.7 and the dielectricconstant of bottom layer 230 (k2) is 18. For example, in Table 1, a toplayer 220 of 0.026 inches and a bottom layer 230 of 0.023 inches yield aread distance of 30 inches. Table 2 represents similar empirical data,where the dielectric constant of layer 220 (k1) is 2.0 and thedielectric constant of layer 230 (k2) is 31.0. Finally, Table 3represents the empirical data when the dielectic constant of layer 220(k1) is 1.2 and the dielectric constant of layer 230 (k2) is 31.0.

TABLE 1 k1 = 1.7 and k2 = 18 Bottom Layer thickness (inches) .017 .023.027 .034 .040 .044 .052 Top Layer .010 0 0 13 12 12 12 20 Thickness.016 20 13 20 19 20 19 20 (inches) .021 13 13 20 20 21 20 21 .026 21 3030 28 33 36 28 .031 26 27 33 28 34 28 28 .037 29 29 39 36 38 24 28 .04320 32 36 38 25 25 28

TABLE 2 k1 = 2 and k2 = 31 Bottom Layer thickness (inches) .023 .033.053 Top Layer .017 12 13 20 Thickness .021 17 19 21 (inches) .034 22 1924 .037 36 20 27 .043 35 35 28

TABLE 3 k1 = 1.2 and k2 = 31 Bottom Layer thickness (inches) .023 .033.053 Top Layer .035 22 28 39 Thickness (inches)

The efficacy of the isolator is influenced by at least two parameters;the total thickness of the isolator and the thickness of each layer.This may be more clearly seen by recasting the data in Tables 1–3 usingthese parameters, as shown in Tables 4–6, respectively. In these tables,the rows represent the total thickness of the isolator in inches, whichis simply the sum of the top layer and the bottom layer from Tables 1–3.The columns represent the proportion of the isolator that is the lowerdielectric constant (tag side) in percent, which is simply the ratio ofthe tag side dielectric to the total isolator thickness.

TABLE 4 k1 = 1.7 and k2 = 18 k = 1.7 Layer Thickness/Total IsolatorThickness, % Total Thickness (inches) 0% 1–9.9% 10–19.9% 20–29.9%30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9% 90–99.9% 100%0.020–0.0399 13 0, 0 13, 20 13 0.040–0.0599 12 12, 12, 20 19, 20, 20 20,13, 30 27, 30, 33 21, 26, 29 0.060–0.0799 20 19, 20, 21 20, 28, 21, 28,28, 34, 36, 38, 39 29, 32, 36 20 36, 33 28, 38 0.080–0.0999 28 28, 28,24, 25 25

TABLE 5 k1 = 2 and k2 = 31 k = 2 Layer Thickness/Total IsolatorThickness, % Total Thickness 0% 1–9.9% 10–19.9% 20–29.9% 30–39.9%40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9% 90–99.9% 100% 0.020–0.03990.040–0.0599 13, 19 12, 17 22 0.060–0.0799 20, 21 19, 20, 35 36, 350.080–0.0999 24 27, 28

TABLE 6 k1 = 1.2 and k2 = 31 k = 1.2 Layer Thickness/Total IsolatorThickness, % Total Thickness 0% 1–9.9% 10–19.9% 20–29.9% 30–39.9%40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9% 90–99.9% 100% 0.020–0.03990.040–0.0599 22 0.060–0.0799 28 0.080–0.0999 39

As shown in these tables, isolator efficacy generally increases withincreasing total isolator thickness but the magnitude of that benefitdepends on the relative proportions of the two layers. For example, inTable 4, a 0.040–0.0599 inch thick isolator yielded a 12-inch read rangewhen the dielectric constant equals 1.7 layer was 10-20% of the totalthickness. This increased to a 30-inch read range when the dielectricconstant equals 1.7 layer was 50–60% of the total thickness.

For performance testing, a tag mounted on metal substrate, with orwithout isolator material interposed between the tag and metal, whensuspended a significant distance from the reader antenna can be subjectto multipath and other RF interference problems from the surroundingenvironment. To overcome this, well-known concepts of performing testsin an anechoic chamber to eliminate RF interference and powerattenuation to simulate changing read distance were used for subsequenttesting.

A test chamber was constructed consisting of a 48″×48″×49½″ box of ¾″plywood oriented with the long dimension being the vertical orientation.In the top surface of the box a centrally located 24″×24″ window wascut. In this window was mounted a 1″ sheet of RF transparent polystyrenefoam and all remaining interior surfaces were covered with Emerson &Cuming Microwave Products, Inc. pyramidal absorber for anechoic chambersECCOSORB® VHP-4. All testing was performed with a ThingMagic Mercury4™reader, antenna and cables. Inside the box, inline with the top window,the ThingMagic Mercury4 antenna was mounted at a distance of 34⅛″ fromthe polystyrene foam window. A test sample to be read was placed on theupper surface of the polystyrene foam, such that the test sample was invertical alignment with the reader at a separation of 35⅛″. The testsample comprised a three element “sandwich”. This sandwich comprised theselected RFID tag in contact with the upper surface of the polystyrenefoam, the test isolator placed over the upper surface of the RFID tag,and a metal plate placed over the upper surface of the isolator. Theantenna configuration was 4 combined UHF Transmit/Receive antennas withcircular polarization. Power attenuation was effected by mean ofsoftware supplied by Rush Tracking Systems, Inc, which instructs thereader. Attenuation was with reference to 32.5 dBm transmission power(30.0 dBm power at the antenna). The reader was instructed to read thetag 20 times and record the percent read rate, that is the number ofsuccessful reads divided by the number of read attempts.

For testing purposes, a minimum 75% read rate was set as the criticalvalue for judging a successful read of the tag at any given powerattenuation. For a given isolator-tag test, the reader determined theread rate at 32.5 dBm transmission power. If the read rate was0%–74.99%, the test condition was recorded as no read (n). If the readrate was 75.00% or greater, the transmission power was decreasedstepwise in 0.50 dB increments until a read rate less than the 75.00%threshold was reached and the corresponding dB attenuation was recorded.As stated earlier, the antenna—tag separation in the test apparatus wasapproximately 35 inches. Thus, a value of 0.0 attenuation, in otherwords, a successful read at 32.5 dBm transmission power, signified aread distance of approximately 3 feet. Successful reading of the tag atincreasing attenuation simulated increasing read distances. Theconversion of attenuation to read distance in feet was not determined.Attenuation values served to compare isolator performance between testconditions.

It is important to note that a value of n signifies that the tag couldnot be read at a distance of 3 feet with a 75% read rate. This does notimply that the combination was inoperable; only that it failed tosatisfy these criteria for the particular test. It is expected that theread distance of all combinations would be better than that of an airgap of the same thickness.

To further demonstrate the improvement of the present invention, tagsfrom two manufacturers were randomly selected for testing. Onecommercially available dipole type tag is the Alien TechnologyCorporation “I2” antenna design, Model ALL-9250 folded dipole. It is aClass 1 Electronic Product Code RFID tag. To verify the improvement ofthe present invention, a tag was randomly selected for testing. Readrates as a function of attenuation, simulating read distance, are shownin FIG. 1 for free space (no proximate metal) and varying air gap spacerfor the tag in proximity to a metal substrate. The read distance of thistest tag in free space with a minimum 75% read rate is approximately10.5 dB attenuation. In proximity to a metal substrate an air gapbetween 0.3–0.4 inches was required to reach a minimum 75% read rate.Even with this air gap, the minimum 75% read rate is attainable at aroughly 3 dB attenuation read distance; a 7 dB degradation from the freespace read distance.

A somewhat more complex antenna design is a crossed dipole design, theSymbol Technologies (formerly Matrics, Inc.) tag, Model X1060, an EPCClass 0 tag. The antenna design and read rates for free space andvarying air gap spacer over a metal substrate are shown in FIGS. 2 a and2 b. The read distance of the tag in free space with a minimum 75% readrate was approximately 9.5 dB attenuation. The air gap required to readover a metal substrate was between 0.3–0.4 inches. Even at this air gap,the minimum 75% read rate was achieved at a read distance of onlyapproximately 1 dB; a degradation of 8.5 dB as compared to the freespace read distance.

Sheets of low loss dielectric isolator materials of various thicknesscomprising the dielectric constants, k, of 2, 4, 9–10, 16, 20, and 30were prepared by blending titanium dioxide with an appropriate siliconeelastomer prepolymer blend and a curing agent, casting a sheet, andcuring the cast sheet either at room temperature or elevatedtemperature, as appropriate to the formulation. Production of suchfilled silicone elastomer is well known in industry. As the dielectricconstant of the formulation is related to the specific gravity,formulations where the dielectric constant is less than about 4 may befoamed with agents such as air and gases, low-boiling organic liquids,chemical blowing agents, or hollow microspheres to reduce the specificgravity. Tables 7–16 illustrate the present invention for two layergradient isolators representing the following combinations:

Lower Higher Dielectric Dielectric Table # Value Value Antenna 7 2 30Alien I2 8 2 20 Alien I2 9 2 16 Alien I2 10 4 30 Alien I2 11 9–10 30Alien I2 12 2 30 Symbol X1060 13 2 16 Symbol X1060 14 2 9–10 SymbolX1060 15 4 30 Symbol X1060 16 9–10 30 Symbol X1060

As was the case in Tables 4–6, in Tables 7 through 16, the rowsrepresent the total thickness of the isolator in inches, and the columnsrepresent the proportion of the isolator that is the lower dielectricconstant (tag side) in percent. The value(s) in each cell record theread distance, in dB, where n means that the minimum 75% read rate wasnot achieved. Where more that one value is listed in a cell, the valuesdo not represent duplicate runs of the same isolator sample. Rather,each value represents a slightly different combination of layers whosetotal isolator thickness and dielectric constant combination conform tothat cell. The isolator material is referenced as k=X/k=Y, where k=Xdesignates the dielectric constant of the layer facing the tag and k=Ydesignates the dielectric constant of the layer interfacing the metalsubstrate. For example, k=2/k=30 signifies a two layer isolator with thetag side being a dielectric material of k=2 dielectric constant and themetal side being a dielectric material of k=30.

It is shown in these Tables that for the Alien I2 tag, with theexception of k=4, Table 10, a single dielectric constant isolator doesnot allow the tag to read, as shown in the 0% and 100% columns. However,a two-layer gradient tag provides tag readability. Tables 7–9 illustratethe effect of varying the high dielectric constant metal side of theisolator while maintaining the low dielectric constant tag side of theisolator at k=2. Tags are readable provided the high dielectric constanttag side is greater than k=16. These Tables further demonstrate that inaddition to the dielectric constants of the isolator, the totalthickness of the isolator and the proportions of the low and highdielectric constant thickness must be considered in selecting theoptimal isolator composition.

TABLE 7 Alien I2 Tag Read Distance, dB attenuation Isolator: k = 2/k =30 k = 2 Layer Thickness/Total Isolator thickness, % Total 1– 10– 20–30– 40– 50– 60– 70– 80– 90– Thickness 0% 9.99% 19.99% 29.99% 39.99%49.99% 59.99% 69.99% 79.99% 89.99% 99.99% 100% 0.040–0.0599 n, n n, n, nn, n, n n, n n, n, n n n n 0.060–0.0799 n n n, n n, n, n n, n, n n n n,n n 0.080–0.0999 n n n, n n n n n, n n, n n, n 0.100–0.1199 n n n, n,1.0 n n n n n n, n n 0.120–0.1399 n n 2.0 n n n, n n, n n n n0.140–0.1599 n n, n, n 0.0, 2.5 n n n n n n 0.160–0.1799 n n, n n, 2.53.5 n, n n n n n n 0.180–0.1999 n n, n, n, n n, 2.5, 4.5 3.5, 7.5 n n, nn, n, n, n n 0.200–0.2199 n n, n, n, n 3.0, 1.0, 0.5, 3.5, n n n, n n,n, n, n n 5.0 3.5

TABLE 8 Alien I2 Tag Read Distance, dB attenuation Isolator: k = 2/k =20 k = 2 Layer Thickness/Total Isolator thickness, % 1– 10– 20– 30– 40–50– 60– 70– 80– 90– 0% 9.99% 19.99% 29.99% 39.99% 49.99% 59.99% 69.99%79.99% 89.99% 99.99% 100% 0.040–0.0599 n, n n, n, n n, n, n n, n n, n,n, n n n n 0.060–0.0799 n n, n n, n n n, n, n n, n n, n n 0.080–0.0999 nn n, n, n, n n, n n n n, n n, n n, n 0.100–0.1199 n n, n n, 2.0 n, 3.5 nn n n, n, n n 0.120–0.1399 n n, n n, 0.5, 2.5 n, 0.5 n, n n n, n n0.140–0.1599 n n, n n, 2.5 2.5 2.5 n, 1.5 n n, n n, n n 0.160–0.1799 nn, n n, 2.5 3.5, 4.5 2.5 1.5 n n, n n 0.180–0.1999 n n, n, n, n, 3.5 3.54.5 2.5 1.5, 3.5 n n, n, n, 0.5 n n, n, n 0.200–0.2199 n n, n 0.0, 3.5,1.5, 4.5 3.5 n, 1.5 0.5, 1.5, 0.5, 0.5, n 4.5, 4.5 3.5 0.5, 1.5

TABLE 9 Alien I2 Tag Read Distance, dB attenuation Isolator: k = 2/k =16 k = 2 Layer Thickness/Total Isolator thickness, % Total 1– 10– 20–30– 40– 50– 60– 70– 80– 90– Thickness 0% 9.99% 19.99% 29.99% 39.99%49.99% 59.99% 69.99% 79.99% 89.99% 99.99% 100% 0.040–0.0599 n n0.060–0.0799 n 0.080–0.0999 n n n n n n, n 0.100–0.1199 n, n n, n, n n nn n 0.120–0.1399 n n n, n n n 0.140–0.1599 n n n n 0.160–0.1799 n n, nn, n n, n n n n 0.180–0.1999 n n n n n 0.200–0.2199 n n, n, n n n n n, nn

TABLE 10 Alien I2 Tag Read Distance, dB attenuation Isolator: k = 4/k =30 k = 4 Layer Thickness/Total Isolator thickness, % Total Thickness 01–9.99 10–19.99 20–29.99 30–39.99 40–49.99 50–59.99 60–69.99 70–79.9980-89.99 90-99.99 100 0.040–0.0599 n, n n, n n n n, 2.5, 2.5 n n0.060–0.0799 n n n, n, n, 2.5 1.5 n n, n, n n n n, n 0.080–0.0999 n n n0.5 0.5 n, 0.5, 1.5 n n n, n n 0.100–0.1199 n n, n n, 0.5 0.0, 0.5 2.51.0, 2.5 1.5 n, 0.0 0.5 0.120–0.1399 n n, 0.5 0.5, 0.5 2.5, 3.5 2.5, 3.52.5

TABLE 11 Alien I2 Tag Read Distance, dB attenuation Isolator: k = 9–10/k= 30 k = 9–10 Layer Thickness/Total Isolator thickness, % Total 1– 10–20– 30– 40– 50– 60– 70– 80– 90– Thickness 0% 9.99% 19.99% 29.99% 39.99%49.99% 59.99% 69.99% 79.99% 89.99% 99.99% 100% 0.040–0.0599 n, n n, n, nn, n n, n n n, n n 0.060–0.0799 n n, n, n, n n n, n n n, n, n, n n0.080–0.0999 n n n, n n, n n, n, n n, n, n n n, n n 0.100–0.1199 n n, nn, n n, n n n n, n n, n, n n 0.120–0.1399 n n n, n n, n, n n, n n, n n,n, n, n 0.140–0.1599 n n, n n n, n n, n, n n, n n n n, n, n, n n0.160–0.1799 n n, n, n n, n, n n n n, n n n n, n, n n, n n 0.180–0.1999n n, n, n, n, n n n, n n n, n, n n, n n, n n 0.200–0.2199 n n n, n, n, nn, n, n n n n n, n n n, n

Tables 7, 10, and 11 illustrate the effect of increasing the dielectricconstant of the low dielectric tag side of the isolator whilemaintaining the high dielectric constant metal side of the isolator atk=30. When the low dielectric constant side of the isolator reached avalue of k=9–10, the tag was no longer readable. Again, consideration oftotal isolator thickness and proportions of the two dielectric constantsin the isolator are important factors in determining the optimalcomposition of the isolator. It is important to note that the last tworows of Table 10 show that an isolator comprising 100% k=4 material canbe read, when the thickness of the material is greater than 0.100inches. The read rates achieved by this homogenous material are notsignificantly different than those achieved by gradient layer isolatorsof the same thickness. However, the gradient isolator also provides tagreadability using a significantly thinner isolator and thereforeprovides an improvement over the single dielectric constant isolator.

It has also been found that the defining limits noted in Tables 7–11 arespecific for a given antenna design. Thus, the optimal composition ofthe isolator material may be different for different antenna designs, asshown in Tables 12–16 for the Symbol Technologies Model X1060 tag.

The response of this tag design to dielectric gradient isolators is ingeneral similar to that of the Alien I2 tag, in that gradient dielectricisolators were more effective then homogeneous isolators. In all cases,the total isolator thickness, the ratio of the thicknesses of the lowand high dielectric constant portions and the actual dielectricconstants of the low and high dielectric portions combine to determinethe effectiveness of the isolator. However, there are differences in theoptimal configuration, which is indicative of the fact that each uniqueantenna design can require different isolation material parameters.

TABLE 12 Symbol X1060 Tag Read Distance, dB attenuation Isolator: k =2/k = 30 k = 2 Layer Thickness/Total Isolator thickness, % Total 1– 10–20– 30– 40– 50– 60– 70– 80– 90– Thickness 0% 9.9% 19.9% 29.9% 39.9%49.9% 59.9% 69.9% 79.9% 89.9% 99.9% 100% 0.040–0.0599 n n n n n n n0.060–0.0799 n n n, n n n n n n 0.080–0.0999 n 2.5 3.5 2.5 1.5 n0.100–0.1199 n 3.5 3.5 7.5 6.5 3.5 n 0.120–0.1399 n 3.5 7.5 5.5 n0.140–0.1599 n 9.5 1.5 0.160–0.1799 n 9.5 2.5

TABLE 13 Symbol X1060 Tag Read Distance, dB attenuation Isolator: k =2/k = 16 k = 2 Layer Thickness/Total Isolator thickness, % Total 1– 10–20– 30– 40– 50– 60– 70– 80– 90– Thickness 0% 9.9% 19.9% 29.9% 39.9%49.9% 59.9% 69.9% 79.9% 89.9% 99.9% 100% 0.040–0.0599 n n 0.060–0.0799 n0.080–0.0999 n 3.5 4.5 7.5 2.5 n 0.100–0.1199 n, n n, n 4.5 3.5 10.5 8.52.5 n 0.120–0.1399 n 5.5 7.5 3.5 0.0 n 0.140–0.1599 n 1.5 0.160–0.1799 n2.5 2.5, 4.5 7.5, 9.5 4.5 2.5

TABLE 14 Symbol X1060 Tag Read Distance, dB attenuation Isolator: k =2/k = 9–10 k = 2 Layer Thickness/Total Isolator thickness, % TotalThickness 0% 1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9%60–69.9% 70–79.9% 80–89.9% 90–99.9% 100% 0.040–0.0599 n n n n n n n0.060–0.0799 n n n, n n n n n n n n 0.080–0.0999 n n n, n n n 4.5 0.0 n,n n 0.100–0.1199 n 0.5 1.5 3.5 3.5 8.5 5   4.5 2.0 n 0.120–0.1399 n 0.5,3.5 4.5 6   7.5 5.5 2.0 n 0.140–0.1599 n 0.0 5.5 6.5 9.5 7.5 7.5 5.5 2.52.5 1.5 0.160–0.1799 n 3.5 5.5 11.5  8.5 6.5 5.5 3.5 2.5

TABLE 15 Symbol X1060 Tag Read Distance, dB attenuation Isolator: k =4/k = 30 k = 4 Layer Thickness/Total Isolator thickness, % TotalThickness 0% 1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9%60–69.9% 70–79.9% 80–89.9% 90–99.9% 100% 0.040–0.0599 n n, n n n, n n, nn n 0.060–0.0799 n n n, n n n, n, n n n n 1.5, 1.5 0.080–0.0999 n n n3.5 3.0, 3.5, 3.5 1.0 n 4.5 1.5 0.100–0.1199 n n 0.5, 3.5 4.5 4.5, 5.0 52.0 0.120–0.1399 n 2.5 2.0, 3.5 3.5 4.5, 5.5 5.5, 0.0 5.5

TABLE 16 Symbol X1060 Tag Read Distance, dB attenuation Isolator: k =9–10/k = 30 k = 9–10 Layer Thickness/Total Isolator thickness, % TotalThickness 0% 1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9%60–69.9% 70–79.9% 80–89.9% 90–99.9% 100% 0.040–0.0599 n n n n n n0.060–0.0799 n n n n n n n 0.080–0.0999 n n n, n n n n n 0.100–0.1199 nn n n n n n n 0.120–0.1399 n n n n n n, n n 0.140–0.1599 n n n n n n 3.5n n 0.160–0.1799 n n, n n n n n 0.5 0.5 3.5 n

Unlike the Alien I2 tag, all of the combinations of low k/high k testedexhibited some thickness between 0.040–0.1799 inches where the isolationwas sufficient to allow the tag to be read. Further, when the isolatorwas at least 0.140 inches thick, even a single dielectric materialisolated the tag when the dielectric constant was less than k=9–10.However, when the low dielectric constant tag side of the isolator wask=2 and the high dielectric constant metal side was k=16 or greater, theadvantage of a gradient dielectric constant isolator over that of asingle dielectric constant isolator was highly significant, typically 7dB or greater read distance.

As described above, the combination of isolator total thickness andproportions of low and high dielectric constant layers that maximizedtag read distance differs between the two antenna designs.

At tag side dielectric constant k=2, the Alien folded dipole tagresponded best to isolators with the k=2 material being 10–30% of thetotal isolator, while the Symbol crossed dipole tag was best in the30–60% range. The crossed dipole tag also yielded significantly greaterread distances, with the tag/isolator/metal equaling the free space readdistance in the best cases.

As an example of an alternate dielectric gradient, isolators wereprepared based on an approximately linear logarithmic gradient of thedielectric constants by sandwiching 0.027-inch thick sheets of theappropriate dielectric constant. Data are shown in Table 17.

TABLE 17 Total Isolator Thickness = 0.137 +/− 0.001 inches Dielectric20% k = 2 20% k = 2 20% k = 2 20% k = 2 Gradient 80% k = 30 20% k = 420% k = 4 20% k = 4 60% k = 30 20% k = 9 20% k = 9 40% k = 30 20% k = 1620% k = 30 Read Distance 4.5 6.5 6.5 3.5 (dB attenuation)

Mirroring the results of two layer dielectric gradients, the best readdistances were obtained when the low dielectric constant tag side of theisolator constituted 30–60% of the total isolator thickness.

FIG. 8 shows a third embodiment of the present invention. While theprevious two embodiments utilized existing RFID tags, FIG. 8 combinesthese existing RFID tags with the present invention to create anintegral solution.

Integrated Circuit 300 is affixed to the top surface of substrate 320,such as by soldering or gluing. This integrated circuit preferablycontains the identifying information related to the article. Substrate320 typically consists of a printed circuit board, although othersubstrates are possible. In most RFID tags, the antenna 310 is affixeddirectly to the substrate 320. In many applications, the antenna 310 iscreated by printing specially sized and spaced wire etches directly onthe substrate 320. Those skilled in the art are familiar with variousprocesses of embedding antennas into printed circuit boards.

Affixed to the opposite surface of substrate 320 is a layer of material330. In one embodiment, the layer of dielectric gradient materialdescribed with respect to FIG. 6 is directly affixed to the bottom ofthe substrate 320. In a second embodiment, two layers of material,similar to those described in connection with FIG. 7, are affixed to thebottom of the substrate 320 such that the layer with the low dielectricconstant is between the substrate and the layer with a high dielectricconstant. As described above, more than two layers can be used to createthe desired dielectric constant gradient. In yet a third embodiment, thedielectric constant gradient is created by applying material directly onthe bottom surface of the substrate 320, such as by printing.

Without being limited to any particular theory, the following offers ahypothesis to explain this result.

As is known by those skilled in the art, most RFID tag antennas for highfrequencies are types or modifications of dipole antennas, since dipolescan be easily printed or etched on a substrate. A well-designed RFID tagwill match the antenna impedance at the terminals to the chip/rectifierimpedance to maximize the power transfer. Antenna performance depends onthe properties of the substrate material. A dipole on a substrate of aparticular dielectric constant will perform differently when put on asubstrate with a different dielectric constant.

The simplest dipole is a strip of conductive material that is one-halfwavelength long at the operating frequency. If this same antenna isplaced on a high dielectric substrate, the operating frequency willchange based on the value of the dielectric constant of the substrate.Also the bandwidth of the dipole, where bandwidth is defined as therange of frequencies for which the antenna will have useful operatingparameters, will decrease. Equations illustrating these phenomena tendto be derived empirically as a rigorous exact solution would becomputationally prohibitive.

Tag antennas are optimized for use in free space or on low dielectricmaterials, such as corrugated cardboard, pallet wood, etc. When an RFIDtag is in proximity to a metallic substrate the impedance of the tagantenna changes. The effect of the metal is to modify the impedance ofthe antenna. This affects the electromagnetic match between the antennaand the chip. The optimum frequency of the antenna will shift (to alower frequency) and the bandwidth of the antenna will be reduced. Thebandwidth becomes important since RFID readers use ‘frequency hopping’or constant changes in frequency around the center frequency to conformto FCC regulations. A reduction in antenna bandwidth would impact readrange based on which frequency is being used at that moment. Further,future standards are expected to require an RFID system to be capable ofoperation at all applicable frequencies without special adaptation, suchthat operation over the full 869 MHz to 953 MHz bandwidth is necessary.The closer the tag is to the metal the greater this mismatch becomes. Ata certain distance, the chip's threshold voltage will not be reached andthe tag will not be read. The distance at which the tag stops reading isdependent on the tag antenna, the chip/rectifier and the reader.

It is believed that the present invention, having material with adielectric gradient, performs two important functions. First, thematerial at the interface with the tag is of sufficiently low dielectricconstant that it does not significantly affect the properties of thesubstrate or the tag antenna. Thus, the tag antenna can maintainfrequency and bandwidth performance. Secondly, the interface with themetal (or high water content) substrate is of a sufficiently highdielectric constant so as to isolate the tag from the article substrateand therefore lessen the modification to the antenna impedance.

Since different RFID tags use different antenna designs, the dielectricgradient material used to insulate the RFID tag from the metal surfacewill be dependent on the tag antenna, chip and reader. Each RFID tagdiffers in antenna type and performance, chip type, reader protocol,etc.

It is therefore expected that it will be necessary to change theparameters of the dielectric constant gradient to optimize the materialfor each tag type. However, the principle of the dielectric gradientisolator is expected to be universal for high frequency RFID tags.

While the use of two or more distinct dielectric constant materials hasbeen shown to improve read distances for a variety of antenna designs,further improvements are possible.

A computer simulation of an idealized tag antenna with backing materialwas performed to gain insight regarding the effect of introducingcomplex magnetic permeability to an RFID tag isolator. Assuming a tagwith IC circuitry with a complex impedance of 10−j60 ohms (where j isthe square root of −1), optimal impedance at the antenna terminals wouldbe the complex conjugate, or 10+j60 ohms. Using Sonnet Software, Inc.electromagnetic analysis modeling software, Version 10.51, the impedanceat the terminals of a 4.4 inch crossed dipole RFID tag antenna withbacking materials of selected electromagnetic parameters and materialthickness mounted on a metallic substrate was modeled. Typical resultsare shown in Table 18 for air, dielectric only and dielectric plusmagnetic backing material.

TABLE 18 ANTENNA TERMINAL IMPEDANCE Backing Thickness, Backing MaterialBacking material inches Air ε = 10 − j0.2 ε = 10 − j0.2 μ = 5 − j1.20.001 0 + j1.4    0 + j3.3 10.4 + j18.4  0.005 0 − j2.9   0.2 + j13.34.7 − j21.1 0.010 0 − j7.3 1.5 + j44 2.9 − j17.7 0.050 0 − j28  2.4 −j62 8.2 − j16.8 0.100 0 − j40  1.8 − j56 14.7 − j20.1 

Simulation demonstrates that the addition of a complex permeability tothe isolator significantly increases the real and imaginary terminalimpedance so as to approach that of the IC circuitry impedance. Also,this effect will be strongly dependent on isolator thickness.

As shown in FIGS. 1 a through 5 a, a multiplicity of RFID tags, withvarying antenna designs and IC circuitry are currently offeredcommercially. In order to satisfy the impedance matching requirementsfor efficient energy transfer from the reader to the tag, a multiplicityof RFID isolator properties must be available. Electromagnetic isolatorsprovide additional latitude in attaining the isolator impedancecharacteristics required to enable RFID tags to be optimally read onmetal substrates. The following examples serve to further demonstratethe advantages of electromagnetic RFID isolator materials.

Electromagnetic isolator material are prepared by blending ferromagneticmaterial with binders, such as plastics or elastomers and forming thinsheets using techniques well known to those skilled in the art. Commonferromagnetic powders suitable for this purpose are iron, nickel, cobaltand their various alloys and ferrites. Isolators of varyingelectromagnetic properties were prepared by blending carbonyl iron withsilicone elastomer prepolymer and a curing agent, casting a sheet, andcuring the cast sheet either at room temperature or elevatedtemperature, as appropriate to the formulation. As described earlier,production of such “filled” silicone elastomer is well known inindustry. Three sample electromagnetic materials were prepared usingthis technique. The measured electromagnetic properties of these exampleisolator formulations at 915 MHz are given in Table 19.

TABLE 19 ELECTROMAGNETIC MATERIAL PROPERTIES Permittivity, εPermeability, μ Formulation A 10 − j0.2 2.7 − j0.4 Formulation B 16 −j0.5 5.0 − j1.8 Formulation C 32 − j1   7.0 − j3.2

To better demonstrate the present invention, electromagnetic isolatorsheets of varying electromagnetic properties and sheet thickness wereprepared by layering selected sheet material. Isolators of homogeneouselectromagnetic material and combined dielectric and electromagneticmaterial were prepared. The isolators were interposed between an RFIDtag and metallic substrate and the tag readability and read range weremeasured.

Commercially available RFID tags representing a range of antenna designswere evaluated. A random sample of each tag style was selected fortesting. The tags, along with the vendor name and model number, arelisted in Table 20 and their design and read range characteristics areshown in FIGS. 1–5.

TABLE 20 EXAMPLE RFID TAGS FIG. Vendor Name Model 1 Alien TechnologyCorp. ALL-9250 “I2” 2 Symbol Technologies X1060 3 Alien Technology Corp.ALL-9354-02 “M” 4 Symbol Technologies Trident 5 Applied WirelessIdentifications, Inc APL-1216

Testing was performed using the same anechoic test chamber and testmethod described above. As was the case above, a minimum 75% read ratewas set as the critical value for judging a successful read of the tagat any given power attenuation (except for Symbol Trident tag tests).

In the following tests, where two layer gradients are utilized, theisolator material will be referred to as kX/Y, where the kX designatesthe dielectric constant of the layer facing the tag and Y designates theelectromagnetic formulation from Table 19 or the dielectric constant onthe layer interfacing the metal substrate. For example, k2/A signifies atwo layer isolator with the tag side being a dielectric material of k=2dielectric constant and the metal side being electromagnetic formulationA with parameters ε=10−j0.2 and μ=2.7−j0.4. An isolator designatedk2/k16 signifies a two layer isolator wherein the tag side is adielectric material of k=2 dielectric constant and the metal side is adielectric material of k=16 dielectric constant.

The Sonnet computer simulation, shown in FIG. 18, suggested thatinclusion of substantial permeability in an isolator, that is,substitution of an electromagnetic material with permeability greaterthan 1 for a material of equal permittivity but permeability of 1, couldprovide isolator efficacy. Two gradient isolators, k2/k16 and k2/B wereprepared with the same total isolator thickness and layer proportion.Note, as shown in Table 19, isolator formulation B has a dielectricconstant of 16 and a magnetic permeabililty of 5.0−j1.8. Test results,shown in Table 21, demonstrate the improved isolator efficacy.

TABLE 21 Read Tag Isolator Properties Distance, dB attenuation Alien “M”tag k2 (0.032″)/k16 (0.084″) no read Alien “M” tag k2 (0.032″)/B(0.080″) 5.5 Symbol X1060 tag k2 (0.035″)/k16 (0.030″) no read SymbolX1060 tag k2 (0.035″)/B (0.030″) 3.5

Further demonstration of the improvement available with the inclusion ofan electromagnetic layer in the isolator is illustrated in Tables 22–25.Tables 22 and 23 again compare two layer isolators where the metal sideis either a dielectric material with a dielectric constant of 16, or anelectromagnetic material having a dielectric constant of 16 withmagnetic greater than 1, specifically Formula B. Tables 22 and 23document read distance as a function of the proportion of the isolatorthickness that is the tag side layer and the total thickness of theisolator for an Alien I2 tag. Tables 24 and 25 show data for a SymbolX1060 tag, comparing two layer isolators whose metal side is either adielectric material with a dielectric constant of 9–10 or anelectromagnetic material having a dielectric constant of 10 and magneticpermeability greater than 1, specifically formulation A.

Regarding the Alien I2 tag, note that the pure dielectric isolator(k2/k1 ) shown in Table 22 fails to provide tag readability over alltest conditions. However, under certain of the test parameters, theisolator with electromagnetic metal side layer (k2/B) allows the tag tobe read, as shown in Table 23. Referring to the 0% column of table 23,note that when the electromagnetic material is at least 0.140–0.1599inches thick, it serves to isolate the tag as a single homogeneouslayer. In contrast, a homogeneous dielectric layer does not provideisolation, as documented in the 0% column of Table 22. The read distanceis maximized with the dielectric-electromagnetic combination isolator,where preferably between 50% and 70% of the isolator is the lowerdielectric constant material. This example demonstrates the improvementof electromagnetic isolators, as either a single homogenous material orin combination with dielectric material, over dielectric isolators forthis tag design. Further, the importance of isolator thickness andproportions of gradient material in determining isolator performance isagain evident.

Regarding the Symbol X1060 tag, both the dielectric gradient isolatorand the dielectric-electromagnetic isolator provide isolation atthicknesses of 0.1799 inch or less. However, the k2/A isolator providestag isolation at a thickness as small as 0.060–0.0799 inches, which canbe advantageous in some cases. It is noted that the k2/k9–10 isolatoroptimizes at a k2 layer proportion of approximately 30–70% and the k2/Aisolator optimizes at a k2 layer proportion of approximately 50–90% ofthe total isolator thickness for this tag.

TABLE 22 Alien I2 Tag Read Distance, dB attenuation Isolator: k2/k16 k =2 Layer Thickness/Total Isolator thickness, % Total 1– 10– 20– 30– 40–Thickness 0% 9.99% 19.99% 29.99% 39.99% 49.99% 50–59.99% 60–69.99%70–79.99% 80–89.99% 90–99.99% 100% 0.040–0.0599 n n 0.060–0.0799 n0.080–0.0999 n n n n n n, n 0.100–0.1199 n, n n, n, n n n n n0.120–0.1399 n n n, n n n 0.140–0.1599 n n n n 0.160–0.1799 n n, n n, nn, n n n n

TABLE 23 Alien I2 Tag Read Distance, dB attenuation Isolator: k2/B k = 2Layer Thickness/Total Isolator thickness, % Total Thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 n n 0.060–0.0799 n 0.5 n 0.080–0.0999 n n, n0.100–0.1199 n n 3.5 n n 0.120–0.1399 n n 2.5, 4.5 3.5 n n 0.140–0.15990.5 4.5, 6.5 5.0 4.0 n 0.160–0.1799 4.5 5.5, 6.0 n

TABLE 24 Symbol X1060 Tag Read Distance, dB attenuation Isolator:k2/k9–10 k = 2 Layer Thickness/Total Isolator thickness, % 1– 10– 20–30– 40– Total Thickness 0% 9.99% 19.99% 29.99% 39.99% 49.99% 50–59.99%60–69.99% 70–79.99% 80–89.99% 90–99.99% 100% 0.040–0.0599 n n n n n n n0.060–0.0799 n n n, n n n n n n n n 0.080–0.0999 n n n, n n — n 4.5 0.0n, n n 0.100–0.1199 n 0.5 1.5 3.5 3.5 8.5 5.0 4.5 2.0 n 0.120–0.1399 n0.5, 3.5 4.5 6.0 7.5 5.5 — 2.0 n 0.140–0.1599 n 0.0 5.5 6.5 9.5 7.5 7.55.5 2.5 2.5 1.5 0.160–0.1799 n 3.5 — 5.5 11.5  — 8.5 6.5 5.5 3.5 2.5

TABLE 25 Symbol X1060 Tag Read Distance, dB attenuation Isolator: k2/A k= 2 Layer Thickness/Total Isolator thickness, % Total Thickness 0%1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9%80–89.9% 90–99.9% 100% 0.040–0.0599 n n 0.060–0.0799 n 3.5 5.0, 6.5 4.5n 0.080–0.0999 n 5.5, 6.5 6.5 4.5, 7.5 n 0.100–0.1199 n 3.5 5.5 6.5 6.58.5, 8.5 6.5 n 0.120–0.1399 n 3.5 — 7.5 8.5, 9.5 — 7.5, 9.5 n0.140–0.1599 — 7.5 8.0 10.5  10.0, 10.5 1.5 0.160–0.1799 8.5 — 9.5 2.5

Tables 26, 27 and 28 compare three two-layer isolators wherein the metalside is composed of electromagnetic material, formulation A, and the tagside is either k2 (Table 26), k4 (Table 27) or k10 (Table 28) dielectricmaterial. The tag is an Alien “M” tag. The electromagnetic material,formulation A, provides isolation as a single layer to this tag, as inthe case of Formulation B, shown above in Table 23. However, in thiscase, the read distance is significantly improved. The isolator ismoderately effective at thickness as small as 0.040–0.0599 inches and issignificantly more effective at 0.100–0.1199 inches thick.

TABLE 26 Alien “M” Tag Read Distance, dB attenuation Isolator: k2/A k =2 Layer Thickness/Total Isolator thickness, % Total Thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 2.5, 3.5 n 0.060–0.0799 2.5 n n 0.080–0.09993.0 4.5, 5.5 n 0.100–0.1199 6.5 7.5 n n n

TABLE 27 Alien “M” Tag Read Distance, dB attenuation Isolator: k4/A k =4 Layer Thickness/Total Isolator thickness, % Total thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 2.5, 3.5 n, 2.5 n n 0.060–0.0799 2.5 4.5, 6.50.5, 1.0 n, n n n n 0.080–0.0999 3.0 7.5 6.5 4.5 n n n 0.100–0.1199 6.56.5 2.0

TABLE 28 Alien “M” Tag Read Distance, dB attenuation Isolator: k10/A k =10 Layer Thickness/Total Isolator thickness, % Total thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 2.5, 3.5 n 0.060–0.0799 2.5 n n n0.080–0.0999 3.0 5.5 5.5 n 0.100–0.1199 6.5 7.5 7.5 8.5 n

These tables further demonstrate the interdependence of electromagneticproperties, total thickness and proportions of material in the gradientin matching the tag antenna requirements. Two additional comparisonswere done; Tables 29 and 30 show the results of combining k=4 dielectricmaterial with Formulations A and B. Tables 31 and 32 show the results ofcombining k=10 dielectric material with Formulations A and C.

TABLE 29 Alien “M” Tag Read Distance, dB attenuation Isolator: k4/A k =4 Layer Thickness/Total Isolator thickness, % Total Thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 2.5, 3.5 n, 2.5 n n 0.060–0.0799 2.5 4.5, 6.50.5, 1.0 n, n n n n 0.080–0.0999 3.0 7.5 6.5 4.5 n n n 0.100–0.1199 6.56.5 2.0

TABLE 30 Alien “M” Tag Read Distance, dB attenuation Isolator: k4/B k =4 Layer Thickness/Total Isolator thickness, % Total Thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 n, n 2.5 n 0.060–0.0799 n, n 5.5 4.5, 4.5 2.50.5 n 0.080–0.0999 n n 2.5, 5.0 4.5, 6.5 6.5, 6.5 3.5 1 n n 0.100–0.1199n 6 7.5, 7.5 2.0

TABLE 31 Alien “M” Tag Read Distance, dB attenuation Isolator: k10/A k =10 Layer Thickness/Total Isolator thickness, % Total Thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 2.5, 3.5 n 0.060–0.0799 2.5 n n n0.080–0.0999 3.0 5.5 5.5 n 0.100–0.1199 6.5 7.5 7.5 8.5 n

TABLE 32 Alien “M” Tag Read Distance, dB attenuation Isolator: k10/C k =10 Layer Thickness/Total Isolator thickness, % Total Thickness 0% 1–9.9%10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9% 80–89.9%90–99.9% 100% 0.040–0.0599 n, n, n, n n 0.060–0.0799 n, n n n n0.080–0.0999 n n n n 0.100–0.1199 n n n

Tables 26 through 32 in concert demonstrate the balancing of parametersrequired for effective isolation of the tag as different isolatorelectromagnetic parameters are selected. When the tag side dielectricconstant is very low, i.e. k=2–4, gradients were superior to thehomogeneous electromagnetic isolator at low isolator total thickness.The locus of effective gradient isolator performance occurred when thetag side dielectric layer was 1–30% of the total isolator thickness.Increasing the tag side dielectric constant to k=10 again demonstratedeffective gradient isolators, but the locus of effectiveness shifted to10–50% of the total isolator thickness being the k=10 tag side layer.

The rebalancing to match antenna requirements is not limited toadjustments on the tag side gradient layer only. Table 29 representsisolator k4/A, while Table 30 represents a k4/B isolator. When the metalside gradient layer changed from electromagnetic material A to materialB, which results in an increase in permittivity and permeability, thelocus of effective isolator performance shifted from tag side dielectriclayer proportion of 1–30% to 20–50%.

The criticality of balancing isolator parameters is demonstrated byTable 31, where isolator k10/A, is compared with Table 32, which has ak10/C isolator. As the metal side gradient layer permittivity andpermeability are increased to the level of electromagnetic material C,the ability to isolate the Alien “M” tag is lost. Without carefulinvestigation over a range of electromagnetic material properties, itmay be erroneously concluded gradient isolators do not perform.

It was found the Symbol Technologies Trident tag was read with lowerread rate by the current ThingMagic4 software provided with the reader.The effect of this can be observed in the free space and air gap readdistance graph, FIG. 4 b. The maximum read rate in free space was onlyabout 10%. With a metal substrate, the tag could not be read until airgap standoff was 0.4–0.5 inches. However, at 0.5-inch air gap standoffthe read rate was 30%, higher than achieved in free space. Based onthese findings, a read rate of 25% was established as the critical valuefor judging a successful read of this tag at any given powerattenuation, as opposed to the 75% value adopted for all other tagtests.

Tables 33 and 34 show the effective isolation of this tag by k2/C andk4/C electromagnetic gradient isolators, respectively. Homogeneoussingle layer isolators of either dielectric material or electromagneticmaterial both provided moderate isolation. However,dielectric-electromagnetic gradient isolators provided significantlygreater read range performance. Further, read range was significantlysuperior to air gap standoff performance.

TABLE 33 Symbol Trident Tag Read Distance, dB attenuation Isolator: k2/Ck = 2 Layer Thickness/Total Isolator thickness, % Total Thickness 0%1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9%80–89.9% 90–99.9% 100% 0.040–0.0599 n, n, n, 1.0 n 0.060–0.0799 n, n nn, n n 1.0 0.080–0.0999 3.0 n n n n, 2.0 2.0 0.100–0.1199 3.0 7.0 7.04.0 n, n 2.0 0.120–0.1399 4.0 8.0, 8.0 1.0

TABLE 34 Symbol Trident Tag Read Distance, dB attenuation Isolator: k4/Ck = 4 Layer Thickness/Total Isolator thickness, % Total Thickness 0%1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9%80–89.9% 90–99.9% 100% 0.040–0.0599 n, n, n, n, n, n n, n n, n n, n, 4.0n n n n n 0.060–0.0799 n, n n, n, n, n, n, 2.0, 6.0 n, 0.0, 5.0, 6.0, n,5.0 n, 5.0 n 3.0 3.5 6.0, 7.0 7.0 0.080–0.0999 3.0 n n, n, 6.0 6.0, 6.0,3.0, 7.0 n, n n 2.0 7.0, 7.0 0.100–0.1199 3.0 8.0 6.0, 6.0 3.0 6.00.120–0.1399 4.0

The Applied Wireless Identifications APL-1216 tag, FIG. 5, was notreadable with the test chamber utilized for read range performancetesting. The possible reason is the minimum read range of the testchamber is approximately 3 feet and the free space read range of the tagis less than that distance. Ignoring this finding, electromagneticisolators were tested for effectiveness. Surprisingly, isolatorparameters were discovered which provided moderate read distance forthis tag, as shown in Table 35. Selection of appropriate isolatorparameters actually enhanced the read range performance of the tag inthis testing.

TABLE 35 AWID APL-1216 Tag Read Distance, dB attenuation Isolator: k10/Ak = 10 Layer Thickness/Total Isolator thickness, % Total Thickness 0%1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9% 70–79.9%80–89.9% 90–99.9% 100% 0.040–0.0599 n n n 0.060–0.0799 n n n0.080–0.0999 n n n, n, 3.5 n 0.100–0.1199 n n 3.5 3.5 n, 3.5 n

In addition to improved isolators comprised of electromagnetic compositematerials as a single layer or in gradient isolators with dielectriccomposite materials, gradient electromagnetic isolators may be utilized.Further electromagnetic isolator material was prepared by blendingferromagnetic carbonyl iron, nickel-zinc ferrite or magnetite withsilicone elastomer prepolymer and a curing agent, casting a sheet, andcuring the cast sheet either at room temperature or elevatedtemperature, as appropriate to the formulation. The measuredelectromagnetic properties of these isolator formulations at 915 MHz aregiven in Table 36.

TABLE 36 ELECTROMAGNETIC MATERIAL PROPERTIES Electromagnetic FormulationFiller Permittivity, ε Permeability, μ D carbonyl iron 5.2 − j0.07 1.5 −j0.1 E nickel-zinc ferrite 4.6 − j0.07 1.7 − j0.3 F magnetite 6.3 −j0.1  1.5 − j0.1

Two-layer isolators utilizing each of the materials in Table 36 as thetag side layer and formulation A as the metal side layer were interposedbetween an Alien M tag and a metal substrate and read distance wasmeasured. Results are given in Table 37.

TABLE 37 Alien “M” Tag Read Distance, dB attenuation Isolator Thickness:0.100–0.1199 inches Tag Side Layer Thickness/Total Isolator thickness, %Isolator 0% 1–9.9% 10–19.9% 20–29.9% 30–39.9% 40–49.9% 50–59.9% 60–69.9%70–79.9% 80–89.9% 90–99.9% 100% D/A 6.5 7.5 — 7.5 5.5 7.5 3.0 2.0 0.0 nn E/A 6.5 7.5 7.0 5.5 5.5 3.0 2.5 2.5 0.5 n F/A 6.5 7.5 — 6.5 6.5 5.54.0 3.0 2.5 n n

For this tag, a gradient electromagnetic isolator wherein the tag sidepermeability is less than the metal side permeability provided someimprovement in read distance when the tag side layer proportion wasbetween 1–20% of the total isolator thickness.

A multitude of RFID tag designs from many manufacturers currently existin the marketplace and the list is expanding as RFID interest increases.As is clear from the tables herein, each tag requires an isolatoruniquely formulated to match the impedance requirements necessary toisolate it from the interfering substrate. This applies not only to tagsfrom different manufacturers, but also to the various tag designs from agiven manufacturer. For example, a Symbol X1060 tag could be readutilizing a k2/A isolator, as shown in Table 25, whereas a SymbolTrident tag was not readable with this isolator profile, but wasreadable utilizing a k4/C isolator, as shown in Table 34. As the antennadesigns are proprietary to each manufacturer and no industry standarddesign exists, identifying an isolator suitable for a given RFID tag isnot possible a priori. The following experimental strategy can providethe appropriate isolator selection.

It can be seen in most of the above graphs that the combination showingthe best performance are not scattered. Rather, there are “clusters” ofoptimal operation. These clusters typically have similar thicknesses andratios of low dielectric material to higher dielectric material. Thisphenomenon can be exploited in developing a strategy that provides theappropriate isolator combination.

Determining the desired isolator information is a two-step process;firstly determining the subset of potential isolator formulations thatcan serve to isolate a given tag design, then selecting the formulationwhich best meets the tag's specified usage. A screening experiment,locating a region on a grid of relative proportions of the variousdielectric and/or electromagnetic components of the total isolatorthickness versus total isolator thickness in the manner of the readdistance tables herein is firstly required. A two-layer isolator seriesis prepared, wherein the tag-side layer consists of a low loss, lowdielectric material whose dielectric constant is 2–4 and thearticle-side layer consists of either a higher dielectric constantmaterial, k=20–30, or an electromagnetic material whose permeability is5–10. An electromagnetic article-side layer is preferred initially.Isolators are fabricated with a total thickness of 0.120–0.0–140 inchesand a tag-side proportion of the total thickness ranging from 0% to 100%in approximately 10% increments. Read distance tests are conducted withthis series of isolators. Other initial screening series may of coursebe utilized should there be other restrictions or specifications placedon the selection of acceptable isolators. The critical element is thatthere be a wide enough screening of formulations such that discovery ofcandidate formulation(s) is relatively likely.

If at least one isolator(s) formulation is discovered that providessufficient isolation of the tag to allow readability, the experimentaltask then becomes one of determining the formulation that satisfies themarket need. This market need can involve such considerations as minimumread distance, thinnest isolator that can meet a given read distance,lowest cost isolator, or any other market criteria. Final isolatorformulation thus involves detailed examination of the region of tagreadability discovered in the first screening experiment by any suitablesearch procedure for the appropriate formulation that best meets themarket need.

If no isolator(s) formulation is discovered that allows tag readability,the next most fruitful screen is to increase the total thickness of theisolator and conduct another series of experiments with isolatorsfabricated with the tag-side layer again ranging from 0% to 100% of thetotal isolator. While any isolator thickness may be selected it ispreferable the thickness be at least 0.240–0.0260 inches, provided theair gap read distance for the RFID tag is greater than this thickness.The second screening thickness must be less than the tag's air gap readdistance to avoid false conclusions about isolator efficacy. If a regionof formulation(s) is discovered that allow readability, determining thebest formulation available to meet market need follows as described inthe preceding paragraph.

Should no viable isolator formulations yet be discovered, changingdielectric and/or electromagnetic properties of isolator layers is next.It has generally been found to be more fruitful to alter the propertiesof the article-side composition firstly both increasing and decreasingdielectric/electromagnetic properties. Screening with a series ofproportions as before and the same greater thickness as the secondscreening is preferable. Success can be limited by the range ofdielectric and/or electromechanical properties that can be achieved withthe raw materials available to the formulator as there are perhaps aninfinite variety of antenna designs possible, but only a finite range ofdielectric and electromagnetic raw materials.

The above strategy assumed the pre-existence of an antenna design, andgenerated a process to determine the optimal isolator combination foruse with that antenna. However, alternatively, the isolator combinationmay be selected first based on cost, thickness, or other factors. Inthis case, subsequent efforts would then be directed toward the creationof an appropriate antenna design for use with that particular isolator.

1. An identification system, adapted to transmit identifying informationabout an article via radio frequency, comprising: an RFID tag, adaptedto transmit said identifying information via radio frequency whenenergized; a first layer having first and second opposite surfaces and afirst dielectric constant, wherein said first surface is adapted to bein contact with said article; a second layer having first and secondopposite surfaces and a second dielectric constant, wherein said secondsurface of said second layer is in contact with said RFID tag, whereinsaid first and said second dielectric constants are unequal.
 2. Thesystem of claim 1, wherein said first dielectric constant is greaterthan said second dielectric constant.
 3. The system of claim 1, whereinsaid first layer comprises titanium dioxide.
 4. The system of claim 1,wherein said second layer comprises titanium dioxide.
 5. The system ofclaim 1, wherein said first surface of said second layer is in contactwith a second surface of said first layer.
 6. The system of claim 1,wherein said first layer has permeability greater than one.
 7. Thesystem of claim 1, wherein said first dielectric constant is from about9 to about
 30. 8. The system of claim 1, wherein said second dielectricconstant is from about 2 to about
 10. 9. The system of claim 1, furthercomprising a third layer, having a third dielectric constant whose valueis greater than said second dielectric constant and less than said firstdielectric constant, wherein said third layer is interposed between saidsecond surface of said first layer and said first surface of said secondlayer.
 10. An identification system, adapted to transmit identifyinginformation about an article via radio frequency, comprising: an RFIDtag, adapted to transmit said identifying information via radiofrequency when energized; and a first layer having first and secondopposite surfaces, a dielectric constant, and a magnetic permeabilitygreater than 1, wherein said first layer is interposed between saidarticle and said RFID tag.
 11. The system of claim 10, wherein saidfirst layer is comprised of carbonyl iron, nickel-zinc ferrite ormagnetite.
 12. The system of claim 10, wherein said magneticpermeability is from about 1.5 to about
 7. 13. The system of claim 10,wherein said dielectric constant is from about 4 to about
 32. 14. Thesystem of claim 10, wherein said first surface of said first layer is incontact with said article and said second surface of said first layer isin contact with said RFID tag.
 15. The system of claim 10, furthercomprising a second layer having first and second opposite surfaces,interposed between said first layer and said RFID tag.
 16. The system ofclaim 15, wherein said second layer has a dielectric constant from about1.5 to about
 10. 17. The system of claim 15, wherein said second layerhas a magnetic permeability greater than
 1. 18. The system of claim 17,wherein said second layer has a magnetic permeability less than thepermeability of said first layer.
 19. The system of claim 15, whereinsaid first surface of said first layer is in contact with said article,said second surface of said first layer is in contact with said firstsurface of said second layer and said second surface of said secondlayer is in contact with said RFID tag.
 20. An identification system,adapted to transmit identifying information about an article via radiofrequency, comprising: an RFID tag, adapted to transmit said identifyinginformation via radio frequency when energized; and a layer, having afirst surface adapted to be in contact with said article, and a secondopposite surface in contact with said RFID tag, wherein said layer has adielectric constant gradient between said first surface and said secondsurface.
 21. The system of claim 20, wherein said dielectric constant atsaid first surface is greater than the dielectric constant at saidsecond surface.
 22. A radio frequency identification system, adapted totransmit identifying information about an article comprising; asubstrate comprising an integrated circuit containing said identifyinginformation and an antenna adapted to transmit said identifyinginformation; a first layer, having first and second opposite surfacesand a first dielectric constant, wherein said first surface is adaptedto be in contact with said article; and a second layer, having first andsecond opposite surfaces and a second dielectric constant, wherein saidsecond surface of said second layer is in contact with said substrateand said first and second dielectric constants are unequal.
 23. Thesystem of claim 22, wherein said first dielectric constant is greaterthan said second dielectric constant.
 24. The system of claim 22,wherein said first layer comprises titanium dioxide.
 25. The system ofclaim 22, wherein said second layer comprises titanium dioxide.
 26. Thesystem of claim 22, wherein said first surface of said second layer isin contact with a second surface of said first layer.
 27. The system ofclaim 22, wherein said first layer has permeability greater than one.28. The system of claim 22, wherein said first dielectric constant isfrom about 9 to about
 30. 29. The system of claim 22, wherein saidsecond dielectric constant is from about 2 to about
 10. 30. The systemof claim 22, further comprising a third layer, having a third dielectricconstant whose value is greater than said second dielectric constant andless than said first dielectric constant, wherein said third layer isinterposed between said second surface of said first layer and saidfirst surface of said second layer.
 31. The system of claim 22, whereinsaid substrate further comprises a first surface upon which saidintegrated circuit and said antenna are located and a second surface incontact with said second surface of said second layer.
 32. A radiofrequency identification system, adapted to transmit identifyinginformation about an article comprising; a substrate comprising anintegrated circuit containing said identifying information and anantenna adapted to transmit said identifying information; and a firstlayer, having first and second opposite surfaces, a dielectric constant,and a magnetic permeability greater than 1, wherein said first layer isinterposed between said article and said RFID tag.
 33. The system ofclaim 32, wherein said first layer is comprised of carbonyl iron,nickel-zinc ferrite or magnetite.
 34. The system of claim 32, whereinsaid magnetic permeability is from about 1.5 to about
 7. 35. The systemof claim 32, wherein said dielectric constant is from about 4 to about32.
 36. The system of claim 32, wherein said first surface of said firstlayer is in contact with said article and said second surface of saidfirst layer is in contact with said RFID tag.
 37. The system of claim32, further comprising a second layer having first and second oppositesurfaces, interposed between said first layer and said substrate. 38.The system of claim 37, wherein said second layer has a dielectricconstant from about 1.5 to about
 10. 39. The system of claim 37, whereinsaid second layer has a magnetic permeability greater than
 1. 40. Thesystem of claim 39, wherein said second layer has a magneticpermeability less than the permeability of said first layer.
 41. Thesystem of claim 37, wherein said first surface of said first layer is incontact with said article, said second surface of said first layer is incontact with said first surface of said second layer and said secondsurface of said second layer is in contact with said substrate.
 42. Aradio frequency identification system, adapted to transmit identifyinginformation about an article comprising; a substrate comprising anintegrated circuit containing said identifying information and anantenna adapted to transmit said identifying information; and a layer,having first and second opposite surfaces interposed between saidsubstrate and said article adapted to be in contact with said article,wherein said layer has a dielectric constant gradient between said firstsurface and said second surface.
 43. The system of claim 42, wherein thedielectric constant at said first surface is greater than the dielectricconstant at said second surface.
 44. The system of claim 42, whereinsaid substrate further comprises a first surface upon which saidintegrated circuit and said antenna are located and a second surface incontact with said second surface of said layer.
 45. A device,comprising: an article having a surface; a first layer in contact withsaid surface of said article, having first and second opposite surfacesand having a first dielectric constant; a second layer having first andsecond opposite surfaces and having a second dielectric constantdifferent from said first dielectric constant; and an RFID tag incontact with said second layer, said RFID tag adapted to transmitidentifying information about said article via radio frequency whenenergized.
 46. The device of claim 45, wherein said first dielectricconstant is greater than said second dielectric constant.
 47. The deviceof claim 45, wherein said first layer comprises titanium dioxide. 48.The device of claim 45, wherein said second layer comprises titaniumdioxide.
 49. The device of claim 45, wherein said first surface of saidsecond layer is in contact with a second surface of said first layer.50. The device of claim 45, wherein said first layer has permeabilitygreater than one.
 51. The device of claim 45, wherein said firstdielectric constant is from about 9 to about
 30. 52. The device of claim45, wherein said second dielectric constant is from about 2 to about 10.53. The device of claim 45, further comprising a third layer, having athird dielectric constant whose value is greater than said seconddielectric constant and less than said first dielectric constant,wherein said third layer is interposed between said second surface ofsaid first layer and said first surface of said second layer.
 54. Thedevice of claim 45, wherein said substrate further comprises a firstsurface upon which said integrated circuit and said antenna are locatedand a second surface in contact with said second surface of said secondlayer.
 55. The device of claim 45, wherein said surface of said articlecomprises metal.
 56. The device of claim 45, wherein said articlecontains liquid.
 57. A device, comprising: an article having a surface;an RFID tag adapted to transmit identifying information about saidarticle via radio frequency when energized; and a first layer havingfirst and second opposite surfaces, a dielectric constant and a magneticpermeability greater than 1, wherein said first layer is interposedbetween said article and said RFID tag.
 58. The device of claim 57,wherein said first layer is comprised of carbonyl iron, nickel-zincferrite or magnetite.
 59. The device of claim 57, wherein said magneticpermeability is from about 1.5 to about
 7. 60. The device of claim 57,wherein said dielectric constant is from about 4 to about
 32. 61. Thedevice of claim 57, wherein said first surface of said first layer is incontact with said article and said second surface of said first layer isin contact with said RFID tag.
 62. The device of claim 57, furthercomprising a second layer having first and second opposite surfaces,interposed between said first layer and said RFID tag.
 63. The device ofclaim 62, wherein said second layer has a dielectric constant from about1.5 to about
 10. 64. The device of claim 62, wherein said second layerhas a magnetic permeability greater than
 1. 65. The device of claim 64,wherein said second layer has a magnetic permeability less than thepermeability of said first layer.
 66. The device of claim 62, whereinsaid first surface of said first layer is in contact with said article,said second surface of said first layer is in contact with said firstsurface of said second layer and said second surface of said secondlayer is in contact with said RFID tag.
 67. The device of claim 57,wherein said surface of said article comprises metal.
 68. The device ofclaim 57, wherein said article contains liquid.
 69. A device,comprising: an article having a surface; a first layer in contact withsaid surface of said article, having first and second opposite surfaces;an RFID tag in contact with said layer, said RFID tag adapted totransmit identifying information about said article via radio frequencywhen energized; wherein said layer has a dielectric gradient such thatthe dielectric constant at the interface of said layer and said surfaceis different from the dielectric constant at the interface of said layerand said RFID tag.
 70. The device of claim 69, wherein said dielectricat the interface of said layer and said surface is greater than thedielectric constant at the interface of said layer and said RFID tag.71. The device of claim 69, wherein said surface of said articlecomprises metal.
 72. The device of claim 69, wherein said articlecontains liquid.